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Subject[PATCH v12 00/11] Guest Last Branch Recording Enabling
Date
Hi all,

Please help review this new version for the Kenrel 5.9 release.

Now, you may apply the last two qemu-devel patches to the upstream
qemu and try the guest LBR feature with '-cpu host' command line.

v11->v12 Changelog:
- apply "Signed-off-by" form PeterZ and his codes for the perf subsystem;
- add validity checks before expose LBR via MSR_IA32_PERF_CAPABILITIES;
- refactor MSR_IA32_DEBUGCTLMSR emulation with validity check;
- reorder "perf_event_attr" fields according to how they're declared;
- replace event_is_oncpu() with "event->state" check;
- make LBR emualtion specific to vmx rather than x86 generic;
- move pass-through LBR code to vmx.c instead of pmu_intel.c;
- add vmx_lbr_en/disable_passthrough layer to make code readable;
- rewrite pmu availability check with vmx_passthrough_lbr_msrs();

You may check more details in each commit.

Previous:
https://lore.kernel.org/kvm/20200514083054.62538-1-like.xu@linux.intel.com/

---

The last branch recording (LBR) is a performance monitor unit (PMU)
feature on Intel processors that records a running trace of the most
recent branches taken by the processor in the LBR stack. This patch
series is going to enable this feature for plenty of KVM guests.

The userspace could configure whether it's enabled or not for each
guest via MSR_IA32_PERF_CAPABILITIES msr. As a first step, a guest
could only enable LBR feature if its cpu model is the same as the
host since the LBR feature is still one of model specific features.

If it's enabled on the guest, the guest LBR driver would accesses the
LBR MSR (including IA32_DEBUGCTLMSR and records MSRs) as host does.
The first guest access on the LBR related MSRs is always interceptible.
The KVM trap would create a special LBR event (called guest LBR event)
which enables the callstack mode and none of hardware counter is assigned.
The host perf would enable and schedule this event as usual.

Guest's first access to a LBR registers gets trapped to KVM, which
creates a guest LBR perf event. It's a regular LBR perf event which gets
the LBR facility assigned from the perf subsystem. Once that succeeds,
the LBR stack msrs are passed through to the guest for efficient accesses.
However, if another host LBR event comes in and takes over the LBR
facility, the LBR msrs will be made interceptible, and guest following
accesses to the LBR msrs will be trapped and meaningless.

Because saving/restoring tens of LBR MSRs (e.g. 32 LBR stack entries) in
VMX transition brings too excessive overhead to frequent vmx transition
itself, the guest LBR event would help save/restore the LBR stack msrs
during the context switching with the help of native LBR event callstack
mechanism, including LBR_SELECT msr.

If the guest no longer accesses the LBR-related MSRs within a scheduling
time slice and the LBR enable bit is unset, vPMU would release its guest
LBR event as a normal event of a unused vPMC and the pass-through
state of the LBR stack msrs would be canceled.

---

LBR testcase:
echo 1 > /proc/sys/kernel/watchdog
echo 25 > /proc/sys/kernel/perf_cpu_time_max_percent
echo 5000 > /proc/sys/kernel/perf_event_max_sample_rate
echo 0 > /proc/sys/kernel/perf_cpu_time_max_percent
./perf record -b ./br_instr a

- Perf report on the host:
Samples: 72K of event 'cycles', Event count (approx.): 72512
Overhead Command Source Shared Object Source Symbol Target Symbol Basic Block Cycles
12.12% br_instr br_instr [.] cmp_end [.] lfsr_cond 1
11.05% br_instr br_instr [.] lfsr_cond [.] cmp_end 5
8.81% br_instr br_instr [.] lfsr_cond [.] cmp_end 4
5.04% br_instr br_instr [.] cmp_end [.] lfsr_cond 20
4.92% br_instr br_instr [.] lfsr_cond [.] cmp_end 6
4.88% br_instr br_instr [.] cmp_end [.] lfsr_cond 6
4.58% br_instr br_instr [.] cmp_end [.] lfsr_cond 5

- Perf report on the guest:
Samples: 92K of event 'cycles', Event count (approx.): 92544
Overhead Command Source Shared Object Source Symbol Target Symbol Basic Block Cycles
12.03% br_instr br_instr [.] cmp_end [.] lfsr_cond 1
11.09% br_instr br_instr [.] lfsr_cond [.] cmp_end 5
8.57% br_instr br_instr [.] lfsr_cond [.] cmp_end 4
5.08% br_instr br_instr [.] lfsr_cond [.] cmp_end 6
5.06% br_instr br_instr [.] cmp_end [.] lfsr_cond 20
4.87% br_instr br_instr [.] cmp_end [.] lfsr_cond 6
4.70% br_instr br_instr [.] cmp_end [.] lfsr_cond 5

Conclusion: the profiling results on the guest are similar to that on the host.

Like Xu (10):
perf/x86/core: Refactor hw->idx checks and cleanup
perf/x86/lbr: Add interface to get LBR information
perf/x86: Add constraint to create guest LBR event without hw counter
perf/x86: Keep LBR records unchanged in host context for guest usage
KVM: vmx/pmu: Expose LBR to guest via MSR_IA32_PERF_CAPABILITIES
KVM: vmx/pmu: Unmask LBR fields in the MSR_IA32_DEBUGCTLMSR emualtion
KVM: vmx/pmu: Pass-through LBR msrs when guest LBR event is scheduled
KVM: vmx/pmu: Emulate legacy freezing LBRs on virtual PMI
KVM: vmx/pmu: Reduce the overhead of LBR pass-through or cancellation
KVM: vmx/pmu: Release guest LBR event via lazy release mechanism

Wei Wang (1):
perf/x86: Fix variable types for LBR registers

Qemu-devel:
target/i386: define a MSR based feature word - FEAT_PERF_CAPABILITIES
target/i386: add -cpu,lbr=true support to enable guest LBR

arch/x86/events/core.c | 26 +--
arch/x86/events/intel/core.c | 109 ++++++++-----
arch/x86/events/intel/lbr.c | 51 +++++-
arch/x86/events/perf_event.h | 8 +-
arch/x86/include/asm/perf_event.h | 34 +++-
arch/x86/kvm/pmu.c | 12 +-
arch/x86/kvm/pmu.h | 5 +
arch/x86/kvm/vmx/capabilities.h | 23 ++-
arch/x86/kvm/vmx/pmu_intel.c | 253 +++++++++++++++++++++++++++++-
arch/x86/kvm/vmx/vmx.c | 86 +++++++++-
arch/x86/kvm/vmx/vmx.h | 17 ++
arch/x86/kvm/x86.c | 13 --
12 files changed, 559 insertions(+), 78 deletions(-)

--
2.21.3

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